Unvented service (G3)
An un-vented cylinder is governed by G3 of the building regulations. It has many safety controls which should be checked on an annual basis or and on a change of a tenancy.
It is also used to satisfy most insurance companies and manufacturers to honour a warranty claim. You may be asked to produce proof of regular maintenance should an incident occur.
Here is a video showing the importance of having a hot water cylinder installed properly, also serviced and checked, as the effects of an unvented hot water cylinder exploding are devastating!